It is a fact that surround and multi-channel sound tracks are gradually replacing stereo as the preferred standard of sound recording. Many new audio devices are equipped with surround capabilities, and most new sound systems sold today are multi-channel systems equipped with multiple speakers and surround sound decoders. In fact, many companies have devised algorithms that modify old stereo recordings so that they will sound as if they were recorded in surround. Other companies have developed algorithms that upgrade older stereo systems to produce surround-like sound using only two speakers. Stereo-expansion algorithms enlarge perceived ambiance, and many sound boards and speaker systems contain the circuitry necessary for delivering expanded stereo sound.
3-D positioning algorithms take matters a step further by seeking to place sounds in particular locations around the listener—to his left or right, above him or below, all in respect to the image displayed. These algorithms are based upon simulating psycho-acoustic cues, replicating the way sounds are actually heard in a 360-degree space. These algorithms often use a head-related transfer function (HRTF) to calculate a sound heard at the listener's ears relative to the spatial coordinates of the sound's origin. For example, a sound emitted by a source located to one's left is first receipted by the left ear and only a split second later by the right one. The relative amplitude of different frequencies also varies, due to the directionality of the pinna and the obstruction of the listener's own head.
As stated above, an HRTF is the measured transformation of sound from a point in space to a specific eardrum. Reproducing the same acoustic information at the ear drums as found in natural free-field listening can create a virtual sound source.
Therefore it is clear that attempts are being made to improve the methods for acquiring HRTF data in order to improve, in turn, the capability to simulate virtual sound sources, using a headphone or speakers. Two of these prior art methods are:    1) using a dummy head with a microphone placed in the location of the ear drums, the dummy head simulating the human head and ears, and    2) placing small microphones inside a subject's ear canal. Due to physical limitations, microphones are placed only halfway into the ear canal.
The measured microphone output represents the individual or dummy head's specific HRTF information. In order to simulate a virtual sound source, the sound signal is convolved with the measured HRTF information.
The above-mentioned prior art methods suffer from the following drawbacks:    1) Since each person has unique HRTF data which represents his unique ears and head sound transformation, the result of using non-individualized HRTF data which was measured using a dummy head or a specific subject, causes a non-satisfactory 3D sensation. This problem affects mostly the higher frequencies, thus causing front-back confusion and an “inside the head” sensation.    2) Another drawback is that the measurements were done near the eardrum, yet the reproduction is done outside the ear, causing the sound to be convolved twice, once using the reference HRTF of the dummy head or specific subject and once using the individual HRTF of the person listening to the headphones. This, of course, causes an inaccurate reproduction of the sound, resulting in an unsatisfactory 3D audio sensation.    3) In order to conduct such an experiment, additional stimulation and measurement equipment must be used. Such equipment (speakers, amplifiers, microphones, etc.) would inevitably influence the measurement by distorting the stimuli and the measured signals. Some components have a linear transfer function, such as the room, the air, the head, the pinnas and ear canals; some have a non-linear transfer function, such as amplifiers, speakers and microphones. A skillful conductor of such an experiment would be able to eliminate the linear influence of the measurement equipment by pre-measuring its frequency response and taking that into account during the analysis. However, current signal-processing techniques are usually unable to eliminate the non-linear portions of equipment distortion.    4) In prior art two-speaker surround systems, the listener must be located exactly between the speakers. Any deviation from that spot results in a distorted sound image.    5) Prior art two-speaker surround systems perform well only in symmetrical environments. The speakers must be matched and the room's acoustics must be symmetrical. This restriction prevents many users from enjoying surround sound over two speakers.    6) Prior art 3D headphone systems provide non-satisfactory 3D sound, mainly causing front-back confusion and an “inside the head” sensation.